Make no mistake, the weather is in general better, but in the winter and fall it can get rainy and cool (or as southern Californians would say, cold). But the humidity is much less, so an 80+ day is really nice while at WDW it could be slightly oppresive.
Defintely worth the trip, especially after Thanksgiving (when Christmas decorations are up and Haunted Manision Holiday and Holiday it's a small world are in full swing) through New Year's, so beginning December should be great. The weekends can get crowded (remember unlike WDW, most vistors are local) but the weekdays should be very nice. Even in the worst case, 3 full days in the parks should get you to just about everything. If the crowds are light, 1.5-2 days can do it (mainly because of the lack of walking and transit time).
When last I saw Main Street, it was looking wonderful. I'm sure you;ll get a kick out of it, and by early December the improved Space Mountain should be open. Definitely worth the trip!
If you do take Southwest, from Chicago you can fly direct to LAX or San Diego. San Diego is a much nicer airport and much easier to navigate. It's about 90 minutes from DL, but there are a couple of good attractions on the way - Sea World, San Diego Zoo (and if you have kids with you)
Legoland.
It's a very easy drive and rental cars tend to be pretty cheap.
If you do fly into Orange County (John Wayne, SNA) note that leaving, the airplane will do something odd (not all pilots mention this). After the initial thrust of take off, they will throttle back the engines, and then a few minutes later throttle up again. This is because homeowners in the OC passed ordinances about the noise of aircraft taking off. If you don't know what's going on, it can be a little disturbing!